1. Field of the Invention
The present invention relates to a magnetic recording medium having a back coat layer. More particularly, the present invention relates to a magnetic recording medium having a back coat layer with excellent durability such that a magnetic layer is not damaged when the magnetic layer is rubbed against the back coat layer.
2. Description of the Related Art
In a magnetic recording medium for use in a video tape or a computer storage device, information is recorded and read via a magnetic head. A tape-type magnetic recording medium run while being in contact with a magnetic head as well as a guide pin, a guide roll, a capstan, a pinch roller and the like in a running path, and therefore, the magnetic layer or back coat layer is easily abraded. In particular, one reason for using the back coat layer is to provide running stability. In a running path of the magnetic recording medium, the back coat layer is often disposed in contact with the guide pins and the like. When the magnetic recording medium is run for a long time, durability of the back coat layer in contact with the guide pins and the like is critical.
It is generally known that inorganic pigment such as alumina having a high Moh's hardness is contained in the back coat layer in order to improve the durability of the back coat layer. For example, Japanese Unexamined Patent Application Publication No. 10-312530 describes a magnetic recording medium comprising a back coat layer containing inorganic powder having a Moh's hardness of 7 or more; 95% or more by number of the inorganic powder having an average particle size within the range of 0.12 to 0.70 μm. Japanese Unexamined Patent Application Publication No. 11-86267 describes a magnetic recording medium comprising a plurality of back coat layers wherein a back coat layer other than the outermost back coat layer, i.e., a middle back coat layer, contains inorganic powder as an abrasive having a Moh's hardness of 5 or more, and being smaller than the film thickness of the middle back coat layer, and the outermost back coat layer substantially contains no inorganic powder having a Moh's hardness of 5 or more.
In recent years, in magnetic recording media for use in computer storage devices, thinner and smoother magnetic layer is needed for electromagnetic conversion properties and improvements in error production as the recording density becomes high. Also, more durable magnetic recording media are needed for increasing the reliability of storage devices.
When alumina is contained in the back coat layer, durability of the back coat layer is improved, but the abrasivity (ability of the back coat layer to scrape an object when the back coat layer is rubbed against the object) becomes high. The magnetic recording medium is wound in a roll within a cartridge, taken out from the cartridge for recording and reading, passed through a running path within the recording device, and again wound in a roll within the device or the cartridge. It has been found that the magnetic layer is scraped by the back coat layer, and the thin, smooth magnetic layer may be scraped by the back coat layer when the magnetic recording medium is wound in a roll many times and the back coat layer has high abrasivity. Such scrapes may cause errors, and fragments of the magnetic layer produced by scrapes may cause clogging in the magnetic head, or may be deposited on the guide pins and the like, resulting in unstable running.